The invention relates to a picture display device comprising a display tube having an evacuated envelope, which envelope comprises, around a longitudinal axis, a display window with a display screen on its inner side, a conical portion and a neck portion, the conical portion being connected to an upstanding wall of the display window.
The invention also relates to a conical portion for use in a picture display device.
Picture display devices of the type described in the opening paragraph are used, inter alia, in television apparatuses and computer monitors and are referred to as cathode ray tubes (CRTs).
A picture display device of the type described in the opening paragraph is known.
The known picture display device has some drawbacks, notably a large weight and high cost price of its conical portion.
It is an object of the invention to provide a picture display device in which said problem is alleviated.
To this end, the picture display device according to the invention is characterized in that, along a diagonal in a direction from the neck portion to one of the corners of the display window, the wall thickness d60% of the conical portion at a distance of 60% between the transition in the conical portion from at least substantially axial symmetry to a lower symmetry and the connection to the upstanding wall of the display window, which distance is measured with respect to the projection on the longitudinal axis, is defined by the relation d60%=dsexe2x88x92xcex1(dsexe2x88x92dpa), in which coefficient xcex1 is in the range of 0.4 less than xcex1xe2x89xa61, and in which dse is the wall thickness of the conical portion at the area of the transition between the conical portion and the upstanding wall, and dpa is the wall thickness of the conical portion at the area of said transition in the conical portion.
Since the wall thickness d60% of the conical portion is thinner than that of the conical portion of the known picture display device, a conical portion comprising less material (glass) is obtained. Due to the reduction of the quantity of material, the picture display device as a whole will be less heavy. A reduction in weight of picture display devices is notably important for picture display devices having a relatively large picture diameter, because such apparatuses can otherwise be hardly lifted. The measure according to the invention may be used to advantage, notably for picture display devices having a large deflection angle ( greater than 100xc2x0). For picture display devices having such large deflection angles, the tensions in the glass of the conical portion are relatively high. To compensate for such high tensions, thicker glass is generally used. Due to the measure according to the invention, such an increase of the quantity of material used is not necessary in picture display devices having a large deflection angle ( greater than 100xc2x0). The weight reduction of the conical portion has the additional advantage that the picture display device as a whole thus has a lower cost price.
The conical portion is provided as a kind of linking element between the neck portion of the known picture display device, which neck portion usually has a relatively small wall thickness, and the upstanding wall of the display window, which upstanding wall has a relatively large wall thickness (inter alia, dependent on the diameter of the display window, the glass composition and requirements imposed on the permeability to X-rays), the wall thickness of the conical portion, measured from the neck portion in the direction of the upstanding wall, generally increasing gradually. The neck portions generally has an axially symmetrical shape with respect to the longitudinal axis, whereas (the upstanding wall of) the display window has a lower, for example at least substantially fourfold symmetry. Viewed from the neck portion, a first portion of the conical portion generally also has a substantially axially symmetrical shape with respect to the longitudinal axis. The (at least substantially axially symmetrical) deflection unit is generally arranged around this first part of the conical portion. A second part of the conical portion has a lower, for example, an at least substantially fourfold symmetry with respect to the longitudinal axis.
The thickness variation of the conical portion is different for the various directions in which the upstanding wall of the display window is reached from the neck portion. Generally, three main directions can be distinguished, namely the thickness variation of the conical portion along a cross-section of the conical portion parallel to the diagonal (to one of the corners of the display window), the thickness variation along a cross-section of the conical portion parallel to the short axis (to the upper or lower side of the display window), and the thickness variation along a cross-section of the conical portion parallel to the long axis (to the sides of the display window). Unless otherwise stated explicitly, the thickness variation of the conical portion in this application is measured in a direction from the neck portion to one of the corners of the display window of the picture display device, which direction is denoted as the direction xe2x80x9calong the diagonalxe2x80x9d.
The relevant area of the conical portion to which said percentage in the above-mentioned relation for d60% is applicable only comprises the second, lower symmetrical part of the conical portion. The thickness dpa=d0% (see FIG. 1B) is the wall thickness of the conical portion at the area of the transition from at least substantially axial symmetry to a lower symmetry in the conical portion, which transition of curvature usually coincides with the end portion of the deflection unit facing the display window. The thickness dse=d100% is the wall thickness of the conical portion at the area of the transition of the conical portion to the upstanding wall of the display window. Generally, the conical portion is connected to the upstanding wall of the display window via the xe2x80x9cseal edgexe2x80x9d. The intermediate values of the wall thickness dx, in which 0xe2x89xa6xxe2x89xa6100%, are determined with respect to the projection of the position on the longitudinal axis. Generally, the variation of the wall thickness dx is gradual. The longitudinal axis is scaled in percents by determining the intersection of the relevant conical portion with projection planes perpendicular to the longitudinal axis. At the position on the longitudinal axis x=0%, the projection plane intersects the transition from at least substantially axial symmetry to a lower, for example an at least substantially fourfold symmetry; at x=100%, the projection plane intersects the transition between the conical portion and the upstanding wall. At the position on the longitudinal axis x=60%, the intersection of the relevant projection plane perpendicular to the longitudinal axis with the conical portion is determined and the relevant wall thickness d60% is measured at this position. Said wall thicknesses dpa, dse and d60% are measured perpendicularly to the curvature at the area of the wall.
A preferred embodiment of the picture display device according to the invention is characterized in that the coefficient xcex1 is in the range of 0.6xe2x89xa6xcex1xe2x89xa61. At values of the coefficient xcex1 in the range xcex1≈0.6, the conical portion can be manufactured (molded) satisfactorily, while a considerable quantity of material is saved. A particularly suitable value for the coefficient xcex1 is xcex1≈0.9. At such high values of the coefficient xcex1, the wall thickness dx of the conical portion in the range of 0xe2x89xa6xxe2x89xa660% is at least substantially the same as the wall thickness dpa at the area of the curvature transition.
In said range of 0.4xe2x89xa6xcex1xe2x89xa61, the picture display device meets the standards imposed on strength and durability of the materials. However, to improve the strength of the conical portion, a coating is provided on the conical portion in the area at which the coefficient is xcex1xe2x89xa70.6. The dynamic fracture behavior of the picture display device is improved thereby. The dynamic fracture behavior is herein understood to mean the fracture behavior upon implosion of the picture display device.
In the case where the measure according to the invention reduces the wall thickness of the conical portion at the area of the transition to the upstanding wall of the display window, it is desirable to reduce the wall thickness of the upstanding wall accordingly. By providing the coating on the upstanding wall of the display window, the desired strength of the upstanding wall is maintained. The coating is preferably provided both on the conical portion and on the upstanding wall of the display window. The coating is usually provided on the outer side of the relevant parts of the picture display device.
A particularly suitable material for use as a coating is a synthetic resin coating, for example a coating of polyurethane. The (polyurethane) coating preferably has a thickness of less than 50 xcexcm. Experiments have surprisingly proved that such a relatively thin layer is already sufficient to maintain the desired strength of the material of the conical portion of the picture display device upon a reduction of the wall thickness of the conical portion according to the invention.